star watch

Star Watch: Days, months, and years go their separate ways

Now that May has begun, the crescent moon is back in the evening sky, becoming thicker and higher each night. It will repeat this behavior in the early days of each month for the rest of the spring and summer, but not beyond. More on that in a bit.

Above the moon this evening, if the sky is clear, you’ll find Jupiter shining prominently. Jupiter this year, and the moon this weekend, pose against the backdrop of the constellation Gemini, the Twins, a lovely grouping whose stars can easily be connected by eye to form two stick figures holding hands.

A month from now, when the crescent moon is back in the western evening sky, Jupiter and Gemini will be nearly gone, sinking low into the sunset. But that won’t disturb the moon. It doesn’t care about what goes on behind it.


It took humanity many thousands of years to realize that. And our calendar still embodies a remnant of the idea that the moon ought to care what other heavenly bodies do.

A monthly delusion

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People have always believed that timekeeping should be simple and rational. Three great time cycles are built into our natural world: the day (one Earth rotation), the lunar month (from new moon to new moon), and the year (one Earth revolution around the sun, creating the seasons). And since people have always felt that they are the center of creation, the creator must have put this setup in place for our use.

The ancient Babylonians paid a lot of attention to the sky and were pretty good at math. They knew that there were about 360 days in a year, which is why today we learn that there are 360 degrees in a circle. A “degree” is the angle the Sun moves each day against the backdrop of the starry universe, fudged a bit to be a nice round number divisible by sixes, tens and twelves, numbers that the Babylonians loved. They also gave us our base-60 numbering for minutes in an hour and seconds in a minute and perhaps our 24-hour division of the day.

But the heavens were messier. There were really about 365 days in a year, but even adding five bonus days to the calendar didn’t keep it in synch with the sun and seasons. In 45 B.C., Julius Caesar, relying on the best astronomers of his day, declared that there would be 365.25 days in a year, creating the leap year to keep the calendar in step with reality. But that wasn’t good enough. By the 16th century, the calendar had drifted 10 days behind the natural seasons. In 1582 Pope Gregory XIII gave us our present Gregorian calendar, with leap-year rules that make the year average exactly 365.2425 days.

That’s still not it. The Gregorian calendar won’t get into trouble for millennia, but the fact is, the annual cycle of the seasons, solstice to solstice, is 365.242190 days. Approximately. The horrible truth was that the day and the year have nothing to do with each other at all. When the world got spinning, there was nobody in charge paying attention. It was as if a clockmaker forgot to connect the minute hand to the hour hand.


The lunar month is the third hand going around the celestial clock. The word month comes from moon. But the moon doesn’t really go around the sky 12 times a year; that’s a convenient fable that people told each other, then fudged their way past. There are actually 12.368266 lunar months in a year, on average.

Nor is the month hand connected to the day hand: a lunar month is 29.530589 days long. Those are messy numbers with essentially random digits.

Therefore, cultures that use actual lunar months in their calendars have a whole additional set of fudge factors and jury rigs to make their dating systems at all workable.

In the western world, the Romans thankfully threw out the lunar hand on the cosmic clock entirely. They decreed that the year would have exactly 12 artificial months, tailored to fit perfectly, and the moon could go its way and be disregarded. This is why we will see the new crescent moon in the evening sky around the same dates for the next few months, and then the moon and months will drift out of phase. Our fake months, so convenient for planning meetings and booking restaurant tables, are 0.906260 days longer, on average, than a lunar month.

We now know the exact reason for these messes. The heavenly bodies were not built as clockwork, but fell together essentially at random as they condensed out of a messy gas-and-dust cloud when the solar system was taking shape. It’s a natural process. Other solar systems around other stars come out differently. What we got was about as fluky as the arrangement of windblown leaves: somewhat predictable in overall pattern, but random in detail. Whenever an intelligent design is what we want, we have to build it ourselves.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge ( His Star Watch column appears the first Saturday of every month.